380 DE. C. CALLiWAY ON THE METAMORPHISM OF [Aug. 1 898, 



and on the small scale, and sheared on the minute scale. Yet, on 

 the other hand, one of the slides (450) ^ from the Pant-y-Glo section 

 is hardly compressed at all, though much of the minute quartz has 

 acquired the granular structure. It is true that the bulk of the 

 rock in this locality is contorted, so that it may be suggested that 

 the heat which took part in the alteration of Xo. 450 was generated 

 by mechanical pressure. But this explanation will not cover the 

 whole of the facts. The Rhosbeirio Shales exhibit signs of only 

 very moderate pressure, yet the matrix has undergone considerable 

 alteration, aud some of the minute quartz has become granulated. 



Fig. 3.— Schist from Pant-y-Glo (No. U7). 



X 60. 



[Alteration still more advanced. A few angular fragments of quartz are 



visible, however, in the contorted meshwork of chlorite and mica.] 



While, therefore, it would seem that mechanical force has been 

 concerned in producing the more intense metamorphism of the 

 lower part of the series, I am not disposed to advance this as the 

 sole cause of the changes produced. This uncertainty, however, 

 does not affect my main thesis — that the Mynydd Mechell schists 

 have been formed by the metamorphism of rocks which were at one 

 time sedimentary strata, and not plutonic masses. 



^ Prof. Bonney was kind enough to examine some of the principal rock- 

 sections of this series, and he expressed his general agreement with my 

 conclusions. The uncompressed condition of No. 450 attracted his special 

 attention, and led him to suggest extreme caution in theorizing. 



